An optical disc, such as a compact disc (CD), is an electronic data storage medium that can be written to and read using a low-powered laser beam. A CD is typically used for electronically recording, storing, and playing back audio, video, text, and other information in digital form. A digital versatile disc (DVD) is another more recent type of optical disc that is generally used for storing and playing back movies because of its ability to store much more data in the same space as a CD.
CDs were initially a read-only storage medium that stored digital data as a pattern of bumps and flat areas impressed into a piece of clear polycarbonate plastic through a complex manufacturing process. Thus, the average consumer often owned a number of CD's that had both desired data (e.g., a few songs they enjoyed on a CD) and undesired data (e.g., the remaining songs on the CD that they disliked). However, average consumers can now put their own data on their own CDs with CD players capable of burning digital data into CD-Rs (CD-recordable discs) and CD-RWs (CD-rewritable discs).
As more and more consumers burn their own CDs, the need for convenient ways to identify the data on such CDs has increased. Methods for labeling the non-data side of optical discs (e.g., CDs, DVDs) with text and images, for example, have continued to develop. Basic methods for labeling a disc include physically writing on the non-data side with a permanent marker (e.g., a SHARPIE marker) or printing out a paper sticker label and sticking it onto the non-data side of the disc. Other physical marking methods developed for implementation in conventional CD players include ink jet, thermal wax transfer, and thermal dye transfer methods. Still other methods use the laser in a conventional CD player to mark a specially prepared CD surface. Such methods apply equally to labeling CDs and DVDs.
A label image can be rendered on the label surface (i.e., the non-data side, or top side) of an optical disc by marking the label surface with a laser beam along concentric circles around the disc. For each circle, spots of constant size and optical density are marked by the laser according to the marking data for that circle. One difficulty in rendering a label image on the non-data side (i.e., label surface) of an optical disc is that there are no markings such as pre-engraved tracks on the non-data side from which radial positioning can be determined. One method for determining radial positioning on the non-data side of an optical disc is the use of a reference pattern on the non-data side. This method is discussed in the related application, “Radial Position Registration for a Trackless Optical Disc Surface”, noted above. Still more accuracy in radial positioning may be required for higher label densities.